Through this highly focused and collaborative effort, we will create and validate new modeling techniques for developing antibodies and vaccine candidates against influenza as a model system for addressing diseases that cannot be addressed by classical exposure to naturally-occurring antigens (Ags). Successful `natural immunity' to viruses induces a protective immune response against disease during subsequent exposure (measles, mumps, rubella, poliovirus, and others) and represents the current state-of-the-art in vaccination technology. Having eradicated many of the scourges of our forebears, the human species now faces new and imminent threats from viruses (such as diverse avian influenza viruses) for which natural exposure does not induce protective immunity to different strains, clades and subtypes. While it has been possible to isolate monoclonal antibodies (mAbs) that inhibit or neutralize specific virus field strains of virtually all human viral pathoges, broad and sustained protection has been elusive. For influenza virus, the core obstacle to sustained immunity is antigenic variability. Many important human pathogens for which there is incomplete immunity and no licensed vaccine exhibit a high level of diversity in circulating field strains. Influenza virus causes yearly epidemics by a relatively slow process of antigenic drift mediated by point mutations in the surface proteins HA and NA. Occasional pandemics, where more rapid shifts are realized by reassortment of surface proteins with zoonotic viruses in birds and pigs, can have devastating consequences due to our immunological navety to novel Ags. Influenza is also a good model system for biodefense: Recent research described adaptations of influenza H5N1 that confer respiratory droplet transmissibility from ferret to ferret, which may mimic the future development of a highly pathogenic pandemic human H5 virus in nature. Current vaccine technologies may not abe dequate to induce the breadth of immunity needed to protect against all drifted or shifted influenza variants. We need to be able to design interfaces so we can control the cross- reactivity of the Abs (or immunogen) induced. To accomplish this goal, we propose to (i) generate an extremely large panel of Abs against diverse influenza HA molecules, (ii) delineate the molecular interactions that confer Ag-antibody (Ag-Ab) recognition, (iii) use these structural data to drive rational Ag design through advanced protein engineering, and (iv) validate designs in vivo, thus producing the next generation of vaccines. The feedback loop, with structural parameterization of Ags and in vitro/in vivo testing of designs, make the protein engineering efforts described here a tractable problem. Rational, structure-informed design also allows us to present native Ags to the immune system on novel scaffolds that are more stable and homogeneous than naturally occurring Ags, which may be flexible and degrade quickly. Our platform also will serve as a model for any biological system (viral, bacterial, and parasitic) or protein-protein design effort, in which it is desirable to control the affinity or specificity of an interaction.